This invention relates generally to cathode ray tubes (CRTs) and is particularly directed to the deposit of a conductive coating of DAG on the inner surface of a CRT funnel.
A conventional color CRT consists of an evacuated envelope having a neck portion, a faceplate, and a funnel portion therebetween. An electron gun is disposed in the neck portion of the envelope, with a tri-color emitting phosphor screen disposed on the inner surface of the faceplate. A shadow mask is located between the electron gun and the screen, in spaced relation from the screen. The electron gun includes a plurality of electrodes for focusing and accelerating three electron beams toward the phosphor screen. The electrodes are maintained in position within the envelope by support means. Typically, several high voltage and low voltage electrodes are attached to the support means along the electron beam paths to facilitate focusing and accelerating of the electron beams. The high voltage electrodes typically operate at an ultor potential of about 30 kilovolts, with the low voltage electrodes typically operating at approximately 8-10 kilovolts, or less.
A conductive coating comprised of a material referred to as "DAG" having a resistance on the order of 100 ohms is disposed on the inner surface of the funnel and neck portions of the envelope. The interior conductive coating operates at ultor potential, with spacers mounted on the electron gun electrode nearest the phosphor screen contacting the interior conductive coating to provide ultor potential to the electron gun. An exterior conductive coating, electrically isolated from the interior conductive coating, is provided on the outside of the funnel to facilitate grounding of the CRT envelope. The interior and exterior DAG coatings on the funnel serve as a large capacitor which filters the high-voltage.
Current practice calls for applying the inner DAG coating manually by inserting a paint brush in the CRT neck and moving it outward toward the flared portion of the CRT funnel as it is rotated. The CRT funnel is then placed on a rotating drum for an application of a DAG coating on its flared portion by means of a stationary airless spray gun as the drum is rotated. This approach is labor intensive and thus expensive; is not conducive to applying a DAG coating of uniform thickness having predictable electrical characteristics; and results in excess DAG not deposited on the CRT funnel which is difficult to remove from, and causes clogging of, the funnel coating apparatus .
The present invention addresses the aforementioned limitations of the prior art by providing for the high speed, automatic spraying of the interior of a CRT funnel with a DAG coating in a precise, high speed, and efficient manner.